In 1981, a remarkable book was published: After Man: A Zoology of the Future, by Dougal Dixon. As a child of the eighties, growing up in a science fiction bubble where daleks, vogons and the fighting machines of the War of the Worlds were at least as concrete to me as anything happening in the real world, After Man presented a biologically-themed alternative world to lose myself in.

The premise of the book is simple: take the Earth today, remove the humans, and let evolution take its course for 50 million years. What new animals evolve? Of course, in other hands this approach could have resulted in a throwaway romp. In Dixon’s, it produced an incredibly detailed, thoughtful book, in which the principles of evolutionary theory and ecology are rigorously applied. Crypsis (adaptations to avoid being seen by either predators or prey) is a common theme, as is mimicry. And convergent evolution (the idea that unrelated organisms in similar ecological niches evolve similar adaptations) is everywhere. Each species has a scientific name which follows the conventions that taxonomists use, and the text describes their behaviours and inter-species interactions. The striking illustrations, with copious annotations, resemble a naturalist’s field notes.

The cover of After Man: A Zoology of the Future Photograph: Breakdown Press

As well as being rigorous, After Man manages to be fun. On newly-formed volcanic islands in the Pacific, bats arrived before birds did, and radiated to occupy new niches. The Flooer, Florifacies mirabila, is one of these new species. The shape and colouring of its head mimics the appearance of a local flower, and its sweet-smelling saliva mimics nectar. It sits on the ground with its face upwards and its mouth open. It is, unsurprisingly, an insectivore.

Dougal Dixon, who studied geology at the University of St Andrews, followed up After Man with two similar speculative biology books. In 1988, The New Dinosaurs: An Alternative Evolution was published, and explored a world where the end-Cretaceous mass extinction hadn’t wiped out the non-avian dinosaurs. Man after Man: An Anthropology of the Future came out in 1990, and speculated on the evolution of our own over the next five million years. There’s a whole generation of evolutionary biologists and palaeontologists who, like me, were inspired by his work. Happily, a new edition of After Man, featuring an updated introduction from Dougal Dixon and a new cover, has just has been published by Breakdown Press, so a new generation can revel in this landmark piece of speculative biology.

The Reedstilt as imagined by Dougal Dixon. Photograph: Breakdown Press

There aren’t many areas of science where speculation is not only tolerated, but can be argued to be essential. Palaeontologists must take an incomplete body of evidence from fossils, and, like Dixon, apply their knowledge of how the natural world works, in order to understand the living organism as it once was. How far that speculation is allowed to expand beyond the existing physical evidence is a judgement call that the researcher, and often the palaeoartists they work closely with, must make. This theme is explored in All Yesterdays, by John Conway, CM Kosemen and Darren Naish, published in 2012. While the palaeontological reconstructions are fascinating, the most enlightening section of this book for me is the reconstructions of modern animals from their skeletons alone, some of which bear no resemblance to their real counterparts. I’ll never look at a baboon in quite the same way again.

Comparative morphology and trace fossils, such as trackways, can tell us a certain amount about behaviour, but without a time machine, interpreting the behaviour of extinct animals is speculative. With soft part preservation rare or absent for most fossils, careful, conservative speculation is essential in reconstructing the appearance of organisms. That’s not to say that bolder speculation isn’t great fun: the reconstruction of elasmosaurs engaging in a neck-swinging competition for mates in All Yesterdays is wholly speculative, but certainly makes you consider all the facets of animal behaviour in the past that we will never be able to know about. And where there are differences between researchers in the degree of speculation, coupled with interpretation of the physical evidence, it can result in ‘interesting’ debates: witness the recent scuffles over a revived theory for aquatic dinosaurs (the #FordvNaish hashtag provides a taster).

Speculative biology has been around ever since HG Wells tackled human evolution in The Time Machine in 1895, but its power to spark our imaginations, as well as inform our understanding of organisms through deep time, is undiminished. Careful, informed speculation is crucial to palaeobiology, and long may it continue.